These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.65 s (backtrack: 0/20).

[ebuild   R    ] app-misc/mime-types-2.1.54::gentoo  USE="nginx*" 29 KiB
[ebuild  N     ] dev-lang/luajit-2.1.1787165859:2/2.1.1787165859::gentoo  USE="-lua52compat -static-libs" 1078 KiB
[ebuild  N     ] dev-lua/lua-resty-core-0.1.30::gentoo  LUA_SINGLE_TARGET="luajit" 222 KiB
[ebuild  N     ] dev-lua/lua-resty-lrucache-0.15::gentoo  LUA_SINGLE_TARGET="luajit" 18 KiB
[ebuild  N     ] acct-group/nginx-0-r2::gentoo  0 KiB
[ebuild  N     ] acct-user/nginx-0-r2::gentoo  USE="-icingaweb2 -mgraph" 0 KiB
[ebuild  N     ] www-servers/nginx-1.31.7:0/1.31.7::gentoo  USE="aio http modules -debug -libatomic -mail (-selinux) -stream -test" NGINX_MODULES_HTTP="access auth_basic autoindex browser charset empty_gif fastcgi geo grpc gzip limit_conn limit_req map memcached mirror proxy referer rewrite scgi split_clients ssi ssl upstream_hash upstream_ip_hash upstream_keepalive upstream_least_conn upstream_random upstream_zone userid uwsgi v2 -addition -auth_request -dav -degradation -flv -geoip -gunzip -gzip_static -image_filter -mp4 -perl -random_index -realip -secure_link -slice -stub_status -sub -v3 -xslt" NGINX_MODULES_MAIL="imap pop3 smtp ssl" NGINX_MODULES_STREAM="access geo limit_conn map pass return set split_clients ssl upstream_hash upstream_least_conn upstream_random upstream_zone -geoip -realip -ssl_preread" 1360 KiB
[ebuild  N     ] www-nginx/ngx_devel_kit-0.3.4::gentoo  65 KiB
[ebuild  N     ] www-nginx/ngx-lua-module-0.10.27::gentoo  LUA_SINGLE_TARGET="luajit" 742 KiB
[ebuild  N     ] app-metrics/nginx-lua-prometheus-0.1_pre20170610-r101::gentoo  LUA_SINGLE_TARGET="luajit" 11 KiB

Total: 10 packages (9 new, 1 reinstall), Size of downloads: 3520 KiB

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by www-servers/nginx-1.31.7::gentoo
# required by www-nginx/ngx_devel_kit-0.3.4::gentoo
# required by www-nginx/ngx-lua-module-0.10.27::gentoo
# required by app-metrics/nginx-lua-prometheus-0.1_pre20170610-r101::gentoo
# required by =app-metrics/nginx-lua-prometheus-0.1_pre20170610-r101 (argument)
>=app-misc/mime-types-2.1.54 nginx